1. Field of the Invention
The present invention relates generally to methods of manufacturing a substrate having a semiconductor on an insulator, and more specifically, to a method of manufacturing an SOI substrate by an SIMOX (Separation by Implanted Oxygen) method.
2. Description of the Background Art
A wafer formed of an insulating substrate and a monocrystalline silicon thin film formed thereon is called SOI (Silicon On Insulator). When a semiconductor device such as an MOS (Metal Oxide Semiconductor) field effect transistor is formed on such a monocrystalline silicon thin film, high speed operation of the device can be achieved by the decrease of parasitic capacitance and the increase of current driving capability, and a short channel effect can effectively be reduced. Conventionally, many approaches have been proposed to form an SOI structure, and the SIMOX method for forming an SOI structure by implanting oxygen ions of a high concentration into a silicon substrate is one of such approaches. Hereinafter, the SIMOX method will be described.
The SIMOX method is a method of forming a buried film (SiO.sub.2 film) directly inside a silicon substrate by implanting oxygen ions into the silicon substrate with an accelerating energy of 200 keV, in a dose amount of 2.0.times.10.sup.18 /cm.sup.2, and then performing an enough thermal treatment at a temperature of 1300.degree. C. or higher in a mixed gas of Ar/O.sub.2 or N.sub.2 /O.sub.2. Now, a method of manufacturing an SOI substrate by a conventional SIMOX method will be described in detail in conjunction with the drawings.
FIGS. 5A-5C are views showing steps in a manufacturing method of an SOI substrate by a conventional SIMOX method.
Referring to FIG. 5A, a silicon substrate 1 is prepared.
Referring to FIG. 5B, oxygen ions 2 are implanted from an upper surface of silicon substrate 1 with silicon substrate 1 being heated at a temperature from 500.degree. C. to 600.degree. C. The implantation condition is, for example, with an accelerating energy of 200 keV in a dose amount of 2.0.times.10.sup.18 /cm.sup.2. The implantation of oxygen ions 2 causes a reaction of silicon substrate 1 and oxygen ions 2, and a buried insulating film (hereinafter referred to as SiO.sub.2 film) 3 is formed. SiO.sub.2 film 3 is formed at a position in a prescribed depth from the upper surface of silicon substrate 1, and therefore a silicon layer 4 is present on SiO.sub.2 film 3.
Referring to FIG. 5C, a heat treatment at a temperature of 1300.degree. C. or higher is performed in an atmosphere of Ar/O.sub.2 for about five hours. Thus, defects produced by the implantation of oxygen ions 2 disappear, crystal quality is recovered, and a monocrystalline silicon layer (hereinafter referred to as SOI layer) 5 is formed as a result. However, since the oxygen ions 2 are implanted in a large amount into silicon substrate 1, various defects result and these defects do not disappear by a heat treatment at a temperature of 1000.degree. C. or higher. Conversely, by a high temperature heat treatment, these fine defects grow to form defects in a line shape which reach from the surface of silicon layer 4 to the boundary surface of silicon layer 4 and SiO.sub.2 film 3. This is called a threading dislocation 6.
Further in order to prevent a reaction with the atmosphere of Ar/O.sub.2 at the time of the above-stated heat treatment, a protecting film (SiO.sub.2) 7 is formed on the surface of silicon substrate 1 as illustrated in FIG. 5D in some cases.
Now, conditions for implanting ions employed in manufacturing an SOI substrate will be described in the following. FIG. 6 is a representation showing the oxygen concentration in silicon substrate 1 relative to the amount of oxygen ions 2 implanted. Assume that the accelerating energy is 200 keV. In FIG. 6, if the amount of oxygen ions 2 implanted is small, oxygen gives a Gauss distribution in silicon substrate 1 and SiO.sub.2 film 3 is not formed in silicon substrate 1. If, however, the amount of oxygen ions 2 implanted is more than a critical implanting amount (1.35.times.10.sup.18 /cm.sup.2) necessary for forming SiO.sub.2 film 3 in silicon substrate 1, the oxygen concentration near the peak of implantation goes beyond the number of oxygen atoms per 1 cm.sup.3 contained in SiO.sub.2, in other words the stoichiometric concentration for SiO.sub.2 (4.4.times.10.sup.22 /cm.sup.3). Therefore, excessive oxygen diffuses toward the leading and trading edges of the distribution and reacts with silicon substrate 1 to form SiO.sub.2, thus providing SiO.sub.2 film 3 having a sharp interface in silicon substrate 1. The reaction at that time is represented as follows: EQU xSi+2Oi-SiO.sub.2 +(x-1)Si.sub.i ( 1)
(O.sub.i : interstitial oxygen, Si.sub.i : interstitial silicon)
Herein, the interstitial oxygen means oxygen atoms which come between interstices and are not coupled to other atoms, and the interstitial silicon means silicon atoms which come between interstices and are not coupled to other atoms. SiO.sub.2 is formed by implanting oxygen ions 2 in an amount more than the critical implantation amount, but the interstitial silicon is discharged in order to restrain the increase in volume which takes place in accordance with the implantation. The interstitial silicon is absorbed into the surface of silicon substrate 1 to be a sink. However, with the increase in the amount of oxygen ion 2 implanted, the number of silicon atoms between interstices produced is increased, and in the meantime excessive silicon atoms between interstices get together to remain as a defect in silicon layer 4. The defect is stabilized as a threading dislocation 6 fixed between the surface of silicon substrate 1 and SiO.sub.2 film 3 in the following heat treatment step, resulting in the degradation of the crystal quality of silicon substrate 1. The mechanism of such dislocation formation is described, for example by J. Stoemenos et al., in J. Appl. Phys., Vol. 69, No. 2, 15 Jan. 1991 pp. 793-802.
The density of threading dislocation 6 depends on ion implantation conditions. FIG. 7 is a representation showing the dependence of the dislocation density in an SOI layer on the amount of oxygen ion implanted and the accelerating voltage. As illustrated in FIG. 7, as the amount of oxygen ion implanted increases and the accelerating voltage is reduced, the dislocation density tends to increase. There exists a method of multi-ion implantation (multi-stages implantation) method which takes advantage of the relativity between the amount of implantation and the density of defects in order to form a high quality SiO.sub.2 film 3 without producing threading dislocation 6. Such a multi-ion implantation (multi-stage implantation) method is reported by D. Hill et al., in J. Appl. Phys., Vol. 63, No. 10, 15 May 1988 pp. 4933-4936. By this method implantation of oxygen ions is performed in a smaller amount (in the range from 0.5 to 1.0.times.10.sup.18 /cm.sup.2) than a conventional method in order to reduce dislocation density, then crystal quality is recovered, SiO.sub.2 is precipitated by a heat treatment, and this implantation and heat treatment steps are repeated a number of times in order to obtain a prescribed amount of implantation. According to this method, an SOI substrate having a quite good quality SiO.sub.2 film with its Si/SiO.sub.2 interface being very sharp and the dislocation density in the SOI layer being 10.sup.3 /cm.sup.2 or smaller is produced. This method however includes a complicated process and is not suitable for commercial mass production.
Another method of reducing dislocation density is proposed by M. K. EL-Ghor et al., in Appl. Phys., Lett., Vol. 57, No. 2, 9 Jul. 1990 pp. 156-158. According to this method, a cavity (hollow space) of a high density is formed in an SOI layer at the time of implanting oxygen ions, and this cavity functions as a sink for interstitial silicon, thus reducing the dislocation density.
In order to use an SOI substrate as a substrate for producing a thin film SOI/MOS field effect transistor, the thickness of the SOI layer should be 1000 .ANG. or smaller. FIG. 8 shows the relation between the thickness of the SOI layer, the amount of implanted oxygen ions, and accelerating voltage. The thickness of SOI layer, as illustrated in FIG. 8, can be reduced as the amount of implanted oxygen ions increases as well as the accelerating voltage is reduced. Any of these conditions however increase the dislocation density, and therefore a method of satisfying the conditions for the thickness of the SOI layer and the dislocation density has not been developed. Furthermore, in steps of ion implantation and heat treatment in an SIMOX method, silicon substrate 1 is contaminated by an impurity from the apparatus.
As described above, in the method of manufacturing the SOI substrate according to the conventional SIMOX method, threading dislocation 6 remains in the SOI substrate or silicon substrate 1 is contaminated with the impurity. Therefore, the crystal quality degrades, and when an MOS field effect transistor is formed on the SOI substrate, the defects or the impurity are taken by a gate oxide film at the time of forming the film. This results in breakdown voltage deficiency and increase in power consumption by current generated by the defects present in the depletion layer, thus deteriorating the characteristic of the device. Furthermore, since a thin film SOI layer of 1000 .ANG. or smaller cannot be provided without letting threading dislocation 6 remain, this method of manufacturing the SOI substrate is not suitable for manufacturing a substrate for producing a thin film SOI/MOS field effect transistor.